“Though your sins are like scarlet”
Though your sins are like scarlet, they will be as white as snow; though they are as red as crimson, they will become like wool. (Isaiah 1:18 BSB)
What it sounds like today
A simple color contrast, red sins turned white.
The cultural key
The force of the image is in the dye, not merely the color. Scarlet and crimson came from the crushed bodies of the kermes insect (the 'crimson worm'), and this was the ancient world's fastest, most permanent dye, valued exactly because it soaked deep into the wool and would not fade or wash out; no bleaching, scrubbing, or sunlight could lift it. So to call sins 'scarlet' and 'crimson' is to call them a dyed-in, indelible stain no human effort could ever remove. God's promise, then, is not that a little red will rinse to pink but that He will do the impossible: take the one stain guaranteed never to come out and make it white as snow, white as clean wool. The modern reader sees a color swap; the first hearers heard God undoing the permanent.
Recovered meaning
Scarlet was a deliberately colorfast dye that could not be washed out, so 'sins like scarlet' are a permanent, indelible stain. God's promise is to remove the unremovable, to make the fixed dye white as snow.